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Kinematic Interpretation of Mylonitic Rocks in Okanogan Dome North-Central Washington and Implications for Dome Evolution

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Kinematic Interpretation of Mylonitic Rocks in Okanogan Dome North-Central Washington and Implications for Dome Evolution University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1983 Kinematic interpretation of mylonitic rocks in Okanogan dome north-central Washington and implications for dome evolution Vicki L. Hansen The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Hansen, Vicki L., "Kinematic interpretation of mylonitic rocks in Okanogan dome north-central Washington and implications for dome evolution" (1983). Graduate Student Theses, Dissertations, & Professional Papers. 7442. https://scholarworks.umt.edu/etd/7442 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. COPYRIGHT ACT OF 1976 Th is is an unpublished m anuscript in which copyright sub ­ s i s t s . Any further r e p r in t in g of it s contents must be approved BY th e a u th o r . Ma n s f ie l d L ibrary Un iv e r s it y of Montana Date : 1 9 R f t KINEMATIC INTERPRETATION OF MYLONITIC ROCKS IN OKANOGAN DOME, NORTH-CENTRAL WASHINGTON, AND IMPLICATIONS FOR DOME EVOLUTION by Vicki L. Hansen B. A., Carleton College, 1980 Presented in partial fulfillment of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1983 Approved by Chair, Board of Examiners D ^ , Graduate School Date UMI Number: EP38243 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Dissertation Publishing UMI EP38243 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQ^sf ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 ABSTRACT Hansen, Vicki L ., M. S., May 1983 Geology Kinematic interpretation of mylonitic rocks of the Okanogan dome, north-central Washington, and implications for dome evolution Director: Donald W. Hyndman Mylonitization and later brecciation of granodiorite record the environment and timing of ductile and b rittle deformation in the Okanogan dome. Deformation and high-grade metamorphism of sed­ imentary rocks was accompanied and followed by emplacement of granodiorite plutons. The paragneiss and granodiorite were mylon- itized, then warped and domally uplifted with coincident develop­ ment of joints, dikes, and chloritic breccia. The myloni te zone dips welst to southwestward and is about 1.5 km thick. Mylonitization increases structurally upward within the dome. Quartz, biotite, and plagioclase record a mylonitic folia­ tion (S) containing a unidirectional elongation lineation. Super­ imposed shear surfaces (C) cut the foliation at an angle of 10- 4 5 0 , the angle decreasing with increasing mylonitization. The angle of inclination of S to C indicates a sense of westward displacement, of upper plate rocks relative to lower plate rocks, in a direction parallel to mylonitic lineation. This sense of shear is opposite to the shear indicated by quartz c-axes and reoriented folds (Goodge, 1983) and stretched inclusions. The mylonitic fabric formed isochemically under middle-green- schist conditions as determined by plotting the percent Ab compon­ ent in coexisting recrystal.lized feldspars. A zone of intense chloritJc breccia disrupts the mylonitic layer­ ing along the west and southwest border of the dome. The zone, about 30 m thick, is subparallel to the mylonitic foliation and confined to the dome margin. Clay gouge, siickensides, and polished surfaces are prominent within the breccia zone. I t is conceivable that the Okanogan mylonite zone experienced earlier easterly directed shear as indicated by reoriented folds and stretched inclusions, followed by later westerly directed shear as indicated by the S and C asymmetry. The ductile mylonitic deformation was followed by more-recent movement which caused brecciation of the mylonitic layering. n ACKNOWLEDGEMENTS I am indfebted to Dave A lt, Brain Atwater, Ken Fox, John Goodge, Don Hyndman, Dean Rinehart, and Steve Sheriff for liv e ly discussions in the fie ld and many helpful suggestions. I extend special thanks to John Goodge and Don Hyndman for insights, encouragement, and patience. I accept full responsibility for the (mis)interpretation within this report, and thank Carol Simpson and Gordon Lister for their challenging remarks which opened my mind to the "truth". This work was accomplished with generous support from the U. S. Geological Survey and the Colville Confederate Tribes, and through research grants from the Geological Society of America (2990-02) and Sigma Xi. m TABLE OF CONTENTS Page ABSTRACT.........................................................................................................................ii ACKNOWLEDGMENTS......................................................................................................i i i LIST OF FIGURES AND TABLES.................................................................................V INTRODUCTION ............................................................................................................. 1 GENERAL GEOLOGY ..................................................................................................... 7 MYLONITIC FABRIC ..................................................................................................... 8 Fabric Interpretation .................................................................................... 13 Megacryst Patterns........................................................................................ 23 CHEMISTRY.....................................................................................................................30 Feldspar Mineralogy ........................................................................................ 32 B iotite Chemistry ............................................................................................ 35 CONCLUSIONS................................................................................................................ 40 TECTONIC FRAMEWORK ............................................................................................ 41 REFERENCES CITED ................................................................................................. 48 IV LIST OF FIGURES AND TABLES Figure Page 1. Metamorphlc core complexes of the North American Cordillera . .2 2. Location map of the Okanogan dome, north-central Washington . .2 3. Generalized geologic map of Omak Lake 15* quadrangle .................. 4 4. Biotite schistose inclusion in megacrystic granodiorite. 10 5. Block diagram of mesoscopic mylonitic fabrics............................... 10 6. Photomicrographs of mylonitic textures ........................................... 15 7. Generalized sketch of Mm and M........................................................... e 21 8. Block diagram of microscopic mylonitic fabrics.......................... 21 9. Sketch of Me and Mm with interpreted sh e a.................................... r 24 10. Models of feldspar megacryst deformation........................................24 11. Model of feldspar megacryst r o ta tio.................................................... n 24 12. Comparision of Mm and Me with lit e r a t u r e.............................................28 13. Comparision of Mm and Me with S and C ................................................... 28 14. Comparision of Mm and Me with Sc and Sm............................................... 28 15. Ternary diagram of feldspar Ab, An and Or components....................34 16. Graph of percent Ab in coexisting recrystal 1ized feldspar . 34 17. Ternary disgram of Fe, Mg and Ti in Me and Mm biotite . .3 7 TABLE I. Rotation of feldspar megacrysts........................................................ .3 7 I I . Whole rock major element analyses..................................................... 38 I I I . Plagioclase mineralogy and chemistry................................................. 38 IV. Orthoclase mineralogy and chemistry......................................................... 38 V. Coexisting feldspars .......................................................................................39 VI. B iotite mineralogy and chemistry..........................................................39 V ADDENDUM In the following report I present a logical arguement for a top-to-the-east sense of shear during the formation of the Okanogan dome mylonitie zone, north-central Washington. Although the argue­ ment appears logical and internally consistent, it is based on the assumption that the major shear plane parallels the mylonitic f o l i ­ ation (Me), marked by compositional layering (page 27). If Me, as I describe i t , formed by stretching RATHER THAN by major shear, my interpretation of the mylonitic fabrics is incorrect. Mm planes would then describe the major sense of shear during formation of the
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